1. Field of the Invention
The present invention relates to an assembling method and an assembling apparatus for assembling mechanical parts by use of a robot, and more particularly to a method and an apparatus for inserting a workpiece into a hole formed in an object or for fitting gear parts together while adjusting gear phase.
2. Description of Related Art
In fitting two members together for example by inserting one member held by a hand attached to a robot arm end into another, if a fitting clearance is small, a method is adopted in which a compliance mechanism is attached to the robot end so as to correct an error in position/posture for insertion of the one member to another. Also adopted is a method of performing a force control during the insertion of one member into another, without using a compliance mechanism, in which a force/moment is detected during the insertion and the robot is operated to decrease the force/moment, whereby a smooth insertion is ensured.
In either case where the compliance mechanism or the force control is used, accurate teaching must be made to enable one member held by the robot to be fitted to the other. For example, in an assembling operation of inserting a rod-like workpiece held by a robot hand into a hole formed in an object, teaching must be made such that a distal end of the workpiece is accurately positioned at the entry of the hole to which the workpiece is to be inserted, because the workpiece cannot be inserted into the hole of the object even if it is pressed toward the hole unless the workpiece is properly positioned relative to the hole, though for which a clearance may be taken into consideration. If the hole is chamfered, an allowable positioning error between the workpiece and the hole increases by an amount of chamfering, and thus the error may be determined based on the clearance and the amount of chamfering.
If the positioning within the allowable error results in failure, the workpiece cannot be inserted into the hole, and in this case, the workpiece and/or the object can be damaged if the workpiece is further pressed against the object.
For the relative positioning between the workpiece and the object, a practical assembling line requires large scaled peripheral equipment such as mechanisms for clamping the workpiece or the object, conveyor mechanisms for transporting and positioning the workpiece or the object, resulting in a high-priced assembling system. In addition, accurate position must be taught to the equipment and robot, which is disadvantageous because a burden is put on a teaching operator.
As a method for automatic positioning, it is known to make a position correction for relatively positioning the workpiece and the hole in accordance with a hole position detected by using a visual sensor or the like before insertion. A method is also known, in which a force or moment is detected to make a search-of the hole position. Still another method using a robot to insert a pin into a hole is known (referring to JP 7-314262 A) where a pin is advanced forward, and it is advanced laterally at random when a force is applied from the direction of advancement of the pin. When a moment acts on the pin, it is rotated in the direction of decreasing the moment. When receiving no reaction force, the pin is advanced straight to be fitted to the hole.
As for the automated search of the fitting position, the method of using a visual sensor entails a problem that the positioning accuracy is insufficient and the cost is high. With another conventional method where the pin is moved laterally at random when it receives a reaction force, a successful search cannot be ensured and a search time can be unduly prolonged.